docs: def Barr_1981(Re, eD):
      r'''Calculates Darcy friction factor using the method in Barr (1981) [2]_
      as shown in [1]_.
  
      .. math::
          \frac{1}{\sqrt{f_d}} = -2\log\left\{\frac{\epsilon}{3.7D} +
          \frac{4.518\log(\frac{Re}{7})}{Re\left[1+\frac{Re^{0.52}}{29}
          \left(\frac{\epsilon}{D}\right)^{0.7}\right]}\right\}
  
      Parameters
      ----------
      Re : float
          Reynolds number, [-]
      eD : float
          Relative roughness, [-]
  
      Returns
      -------
      fd : float
          Darcy friction factor [-]
  
      Notes
      -----
      No range of validity specified for this equation.
  
      Examples
      --------
      >>> Barr_1981(1E5, 1E-4)
      0.01849836032779929
  
      References
      ----------
      .. [1] Winning, H. and T. Coole. "Explicit Friction Factor Accuracy and
         Computational Efficiency for Turbulent Flow in Pipes." Flow, Turbulence
         and Combustion 90, no. 1 (January 1, 2013): 1-27.
         doi:10.1007/s10494-012-9419-7
      .. [2]	Barr, Dih, and Colebrook White."Technical Note. Solutions Of The
         Colebrook-White Function For Resistance To Uniform Turbulent Flow."
         ICE Proceedings 71, no. 2 (January 6, 1981): 529-35.
         doi:10.1680/iicep.1981.1895.
      '''
      fd = (-2*log10(eD/3.7 + 4.518*log10(Re/7.)/(Re*(1+Re**0.52/29*eD**0.7))))**-2
      return fd